Synopsis
Graeme Willy and Clive Gollings (Simon Pegg and Nick Frost) are two British comic book enthusiasts and best friends who have traveled to the United States to attend the annual San Diego Comic-Con International and to take a road trip in their RV to visit sites of major extraterrestrial importance. When stopping for something to eat they meet two rednecks and when Graeme laughs with them they start tormenting him. They leave quickly and dent the rednecks' truck. At night, along the highway, they notice lights following them. Wrongly thinking the lights to be from the rednecks' dented truck they speed up, but it's actually a car that passes them and then crashes. They investigate the damaged car, and discover an alien named Paul (voiced by Seth Rogen), who is in desperate need of help.
Although shocked by the appearance of Paul, Graeme agrees to give Paul a ride, but Clive (who fainted after seeing Paul) is displeased about not being consulted about the idea. Later, United States Secret Service Agent Lorenzo Zoil (Jason Bateman) arrives at the site of the crashed car and informs his mysterious female superior, called the "Big Guy", over the radio that he is closing in on Paul, and she recommends using local law enforcement as back-up. She then recruits two inept rookie agents, Haggard (Bill Hader) and O'Reilly (Joe Lo Truglio), to aid in his mission, without informing them about the nature of their target. Zoil assures the Big Guy he can handle the problem himself, but the rookies are employed anyway.
Graeme, Clive and Paul pull into an RV park run by Ruth Buggs (Kristen Wiig), a Christian fundamentalist, and her over-bearing father, Moses (John Carroll Lynch). The trio bond around their camp grill and Paul reveals that since he was captured by the government, he had been advising them in all manner of scientific and sociological achievements. Having divulged everything that he knows, Paul has outlived his usefulness as a receptacle of knowledge, thus his captors intended to surgically remove Paul's brain and harvest his stem cells, in an attempt to harness his physical abilities (Paul had previously demonstrated both invisibility and healing powers). With the help from a friend inside Area 51, Paul sent an SOS to his home planet and was escaping to rendezvous with them.
The next morning, Paul intentionally reveals himself to Ruth during a theological discussion which turns into an argument, and the trio are forced to kidnap her and make a hasty escape. Moses sees Paul while the alien is trying to collect Clive's passport (that he left with Ruth after parking), and, believing him to be a demon, grabs his shotgun and chases after them in his truck. Paul shatters Ruth's faith by sharing his knowledge of the universe via a telepathic link; at first horrified, Ruth suddenly becomes eager to sin, which her father had raised her to fear doing. She initially does not trust Paul, but he heals her left eye, from which she was blind since the age of four.
When the foursome stop at a bar to rest, Ruth tries to make a phone call to her dad, but Zoil answers, as part of the operation to capture Paul. At the end of the phone call, in which Ruth gives no information, she runs into the rednecks, who show an ill-received interest in her. She evades them, and informs Graeme and Clive they need to leave. At this point, the rednecks run into the men again, and a fight starts. Moses is also at the bar, and he sees Ruth, but notices her eye. As he is so stunned, he fails to react and loses track of the trio as they dash for the RV. The rednecks chase them down, but Paul appears outside the RV. The two rednecks faint and they drive away. Later that night, Paul, Graeme, Ruth and Clive sit down together near an RV park. Ruth falls asleep but goes back to the RV. In the morning, Graeme and Clive find it difficult to hide Paul from a busy town. Paul goes into a comic book store with Clive as Graeme goes to pick up Ruth. Paul has to pose as a statue there while they wait.
Meanwhile, Ruth was asked questions by Agent Zoil, but lied and said she knew nothing of a one-eyed girl or two British "nerds". She and Graeme then drive into town to pick up Clive and Paul, but Paul had been exposed after he was caught by O'Reilly. Clive and Paul jump into the RV and narrowly escape the town; Haggard and O'Reilly report to Zoil. The rookies are asked to return to the base, but they want to catch the alien themselves. Moses is also in town, still trying to steal Ruth back, so he chases the RV. Clive and Graeme stop to buy fireworks at a shop but end up stealing them after Clive realizes they have no more American money.
Paul eventually reveals his intention to return to Tara Walton (Blythe Danner), a girl whose dog he crashed his ship on in 1947 and who subsequently saved his life, who is now an old woman. After spending her life being ridiculed for what she said she saw, Tara is grateful to see that Paul exists. She turns her gas cooker on to make tea, but is interrupted by Haggard and O'Reilly on one side of the house, and Zoil on the other. As the motley crew escapes and drives off with Paul, O'Reilly shoots at them, and the gas ignites, destroying Tara's house and seemingly killing O'Reilly. A winded Zoil attempts to follow, but Haggard takes off first, shooting at Moses, running him off the road and catching up to the RV. However, due to an error in judgement, Haggard accidentally drives off a cliff and is killed, leaving Zoil as the last remaining agent in pursuit. He reassures the Big Guy that he will have Paul within an hour, but she declares herself tired of waiting, and informs Zoil that she has ordered a military response.
When Paul, Graeme, Clive, Ruth and Tara arrive at the rendezvous, they set off a signal and wait. Eventually, eerie orange lights show up over the surrounding trees, and everyone believes that it is Paul's rescue ship. However, it is an army helicopter, with the Big Guy (Sigourney Weaver) on board. As she and three troops move to shoot Paul, Zoil arrives, and it is revealed that he was Paul's inside contact who helped him to escape. Zoil disarms the men, but is shot in the shoulder by the Big Guy. Tara punches out the Big Guy, but Moses appears with his shotgun and mortally wounds Graeme while aiming at Paul. Paul heals him and collapses, briefly appearing to be dead before he quickly recovers; Moses proclaims this to be a miracle from God. The Big Guy regains consciousness, but is immediately crushed to death by the arriving alien ship. Paul begins to depart and informs Tara that she is coming with him to live a better life. He bids farewell to his friends, hoping to meet them again one day.
Two years later, Graeme, Clive, Ruth, and O'Reilly, who is revealed to have survived the explosion, are shown again at the Comic-Con convention, where Graeme and Clive are promoting Paul, their new hit novel.
